We are continuing our studies on porphobilinogen synthase, an octameric enzyme, which catalyzes the synthesis of the pyrrole, porphobilinogen. The pyrrole polymerizes to heme, chlorophyll and the corrin ring of vitamin B12. Our studies planned for this year are concerned with obtaining more details on the nature of the active site, the number of active sites and further details as to the mechanism of synthesis. The primary sequence, when determined, will be of great help in the understanding of the structure-function relationships. Along with these studies, we shall continue our subunit-subunit hybridization studies using our previously described immobilized enzyme preparation. This preparation allows us to study interspecies and intraspecies hybridization, and thus allows studies of structure-function relationships.